Image sensors find applications in a wide variety of fields, including machine vision, robotics, guidance and navigation, automotive applications and consumer products. In many smart image sensors, it is desirable to integrate on-chip circuitry to control the image sensor and to perform signal and image processing on the output image. Charge-coupled devices (CCDs), which have been one of the dominant technologies used for image sensors, however, do not easily lend themselves to large scale signal processing and are not easily integrated with complimentary metal oxide semiconductor (CMOS) circuits.
CMOS image sensors receive light into an imaging array including a photosensitive pixel array. One of the difficulties in designing imaging systems is in the optimization of individual pixels within the pixel array. The problem becomes significant in imaging applications receiving non-telecentric light, where different pixels of the array are illuminated with light having different chief ray angles (CRAs). Examples of non-telecentric light sources include cameras for cellular phones and imaging handsets.
If the incident light is provided perpendicular to the CMOS image sensor (corresponding to a CRA of about 0°), the sensor has full sensitivity to the incoming light. However, if the incident light is provided at an angle (i.e. where the CRA angle is greater than 0°), the image sensor may not capture all the light. For example, in non-telecentric applications, pixels at the corner of the imaging array may typically receive light with CRAs up to about 30°. In addition, the relative illumination of the imaging lens results in large (up to about 50%) degradation of light intensity across the array. The signals output from pixels at an edge of the array may be significantly degraded. A mismatch between an acceptance angle for the incident light and the incident ray angle may cause vignetting (i.e. a corner darkness effect or shading effect) to occur.
Obtaining an optimized imaging array is becoming increasingly important as technology tends towards producing a reduced pixel size along with an increased image quality. In addition, many imaging lenses for mobile applications, such as cellular telephones, have a significant non-linear dependence on the CRA which is a function of the positions of pixels in the pixel array. Accordingly, there is an interest in determining a profile of chief ray angles in order to optimize the imaging array and optimize signal processing correction parameters.